1. Field of the Invention
The present invention relates to a thermal head control circuit and a thermal head control method for controlling a thermal head for developing color on thermosensitive recording paper, and more particularly to a thermal head control circuit and a thermal head control method permitting multicolor printing.
2. Description of the Prior Art
FIG. 1 illustrates the configuration of a conventional thermal head control circuit. In the figure, a thermal head has a plurality each of heating resistor elements 600a to 600c and transistors 700a to 700c. A print control section 100 receives print information from a main control section, such as the control unit of a printer (not shown), supplies a head control section 200 with print data, indicating whether or not each dot on one line should be or need not be printed, as well as a latch signal S2. The head control section 200, in accordance with the print data, turns on or off the transistors 700a to 700c respectively connected to the heating resistor elements 600a to 600c constituting individual dots of the thermal head.
Next will be described the operation of this prior art thermal head control circuit.
The print control section 100, having completed outputting of print data indicating individual dots on one line to the head control section 200, supplies the latch signal S2 to the head control section 200. The head control section 200 has a shift register for shifting, for instance, print data supplied from the print control section 100, latches the print data in accordance with the latch signal S2, and outputs them in parallel. This causes the transistor, out of the transistors 700a to 700c, corresponding to each dot supposed to be printed out of the print data, to be turned on. A current flows to a heating resistor element through the transistor turned on, and the heating resistor element emits heat. Accordingly, each dot to be printed on the thermosensitive recording paper is printed.
The following description refers to a case in which such a thermal head control circuit is applied to a small size thermal printer and dichroic printing is accomplished. Dichroic printing is usually accomplished by varying color development according to a difference in calorific value (energy) given to thermosensitive recording paper.
The calorific value (energy) is proportional to the duration of the electrification of the heating resistor elements.
Therefore, when second color development is to be accomplished in dichroic printing, a greater calorific value needs to be applied than in first color development, and the transistors 700a to 700c need to be kept electrified for a longer duration. In this case, where high speed printing is to be done, the duration of electrification may prove too long to be accommodated in one line step of recording paper feeding. This problem can be solved, as described in the U.S. Pat. No. 4,983,054, by reducing the paper feeding speed for the second color development.
Incidentally, not only in dichroic printing, color development can be controlled by regulating the duration of electrification in usual adjustment of density as well.
The conventional thermal head control circuit, being composed as described above, involves the problem that the printing speed is reduced especially when printing in multiple colors. Moreover, when multicolor printing has to be done on a dot-by-dot basis within one line, a plurality of different durations of electrification should be handled, resulting in complexity of processing by the head control section.